1. Field of the Invention
The present invention relates to a method and device for testing a semiconductor laser used for, for example, optical fiber communications, and more particularly to a method and device for testing a distributed feedback semiconductor laser having a phase shift diffraction grating.
2. Background Art
Distributed feedback semiconductor lasers have hitherto been frequently used for optical fiber communications due to its capability in realizing single longitudinal mode oscillation. Above all, distributed feedback semiconductor lasers which have λ/4 phase shift diffraction grating having a phase shift part corresponding to the quarter wavelength in a part of the diffraction grating are in heavy usage in the high-speed optical communication field due to its high yield and excellent stability in single longitudinal mode oscillation (for example, refer to T. Numai, “Fundamentals of Semiconductor Laser Engineering”, pp. 167–170, published 1996 by Maruzen Co., Ltd.).
In optical fiber communications and especially in long-distance optical fiber communications, a light having a wavelength equal to a band of 1.55 μm is used. This is because 1.55 μm is the wavelength at which least absorption loss of light occurs in an optical fiber.
However, in the band of 1.55 μm, the wavelength dependence of the refractive index of the optical fiber is large compared with other bands of light. This means that the speed of light in the optical fiber has a wavelength dependence.
Generally, if a modulation current is applied to a semiconductor laser, a change in the oscillation wavelength called ‘chirping’ occurs. In the case where the wavelength of the laser changes dynamically according to the chirping, the time of arrival of signal light shifts because of the above-mentioned wavelength and dependence, and deformation in modulating signal waveform occurs. The degree of deformation becomes more drastic as the transmission distance becomes longer, and this disables the signal to be read accurately at the receiving side. For this reason, the transmission distance is limited by the chirping.
Chirping occurs on the principle mentioned below. At the time of modulation of the laser, the carrier density inside the laser changes. Due to the change in carrier density, a change in the refractive index occurs (this phenomenon is known as ‘plasma effect’).
On the other hand, the oscillation wavelength of the distributed feedback laser is practically defined by the wavelength λ which satisfies the Bragg condition. Here, the following formula may be given:λ=2×n×m×Λ  (1)where: n is the refractive index, m is an integer not smaller than one, and Λ is the period of the grating. Accordingly, it may be understood with facility that the wavelength changes depending on the change in the refractive index. The wavelength of the laser changes at modulation on this principle.
It is known that the amount of chirping depends on the amount of phase shift in the distributed feedback laser (for example, refer to Y. Huang et al., “Isolator-Free 2.5-Gb/s 80-km Transmission by Directly Modulated λ/8 Phase-Shifted DFB-LDs Under Negative Feedback Effect of Mirror Loss”, IEEE Photonics Technology Letters, vol. 13, No. 3, pp. 245–247).
The amount of phase shift is normally set at quarter wavelength (½ of the grating period) for stable single longitudinal mode oscillation, however, since the grating period is as small as 0.2 to 0.25 μm, in the actual element-manufacturing process, the phase shift amount deviates from the original design value λ/4, and a variation occurs. As a result of this, a variation occurs also in the amount of chirping among the elements, and to guarantee the transmission characteristics of the elements, it is necessary to conduct a 100% test.
The transmission characteristics of an element are generally dealt with using a parameter called ‘power penalty’. Upon guaranteeing the transmission characteristics of a semiconductor laser product, in most cases, the standard is set based on the value of the power penalty. Power penalty is a parameter for indicating the degree of deformation of modulating signal waveform, and is defined by the difference of receiver sensitivity (input optical power to the receiver) that is necessary for realizing the given code error rate in the specified transmission distance and in 0 km.
Accordingly, to test characteristics of transmission to elements means to measure power penalty, i.e., to measure the code error rate before and after transmission while varying the input optical power to the receiver.